Implement graph export method on Node

An export method is added on the Node class which returns a json object
that represents the graph of the node instance and its definitions,
recursively. The return value is as speficified in issue #38 with a
format version number, the id of the top-level node, and dicts of nodes
and relationships, each with an arbitrary but consistent id, a type, and
a dict of properties. To support this method, __eq__ and __ne__ methods
are added to the Node and Rel classes and a get_map_id utility method is
added (in utils) that iterates over {`id`: `obj`, ...} type dicts and
returns the id of the passed object or a new, unclaimed id and a boolean
representing whether or not the id is new as a two-dict.

Fixes #38
Fixes #54

Signed-off-by: Aaron Browne <aaron0browne@gmail.com>
Never use mutable objects as default arguments:

```python
def f(key, a={}):
    a[key] = 1
    return a

f('foo')  # {'foo': 1}
f('bar')  # {'bar': 1, 'foo': 1}
```